This invention relates to sintered bodies prepared from synthetic cordierite.
The industrial importance of cordierite (2 MgO:2 Al.sub.2 O.sub.3 :5 SiO.sub.2) lies in the fact that bodies made of cordierite are characterizable as having a very low thermal expansion over a wide range of temperatures, a high thermal shock resistance when subjected to abrupt temperature changes, and very low dielectric and loss constants at high frequencies. Typically, cordierite bodies may have thermal expansion as low as 0.5.times.10-6/.degree.C. in the range of 0.degree. C. to 500.degree. C. and dielectric constants as low as 4.8-5.0 at frequencies of 1 MHz. Industrial uses for cordierite, therefore, include refractory heat plates, honeycomb structures for cleaning exhaust gas from automobiles, radomes and electronic circuit substrates.
The use of cordierite in radomes and electronic circuit substrates takes advantage of the low dielectric constant of cordierite. However, many known cordierite compositions contain impurities, such as alkali metals and/or transition metals, which degrade dielectric properties. Typically, these impure cordierite compositions are those prepared from natural starting materials such as talc and clay. The preparation of cordierite from natural starting materials is disclosed in, e.g., U.S. Pat. Nos. 2,599,184; 3,885,977; 3,958,058; 3,967,971; 4,125,408; 4,194,917; 4,268,311; 4,434,117; and 4,476,236.
Other processes for the preparation of cordierite use, e.g., mixed oxides, glass, or alkoxides or mixtures thereof as the starting material. These approaches do not necessarily require alkali metals or transition metals. However, one of the chief difficulties associated with pure cordierite is its extremely poor sinterability. The approach usually taken to overcome the poor sinterability of pure cordierite is the addition of one or more additional components which either promote liquid phase sintering or, in the case of glass powders, may lower the viscosity of the powder. Almost all the alkali and alkaline earths have been investigated as sintering aids. However, it is generally recognized that small amounts of calcium oxide, also called calcia, are undesirable or have no effect. For example, U.S. Pat. No. 3,926,648 discloses at column 3 that a glass composition based upon the cordierite stoichiometry, to which was added 0.95 percent calcia, resulted in a sintered body which exhibited only from 1-3 percent shrinkage and "poor" strength. U.S. Pat. Nos. 3,979,216 and 4,235,855 disclose cordierite ceramic bodies wherein the total concentration of the alkaline earths calcium, strontium and barium is required to be below about 600 ppm, and below about 200 ppm per individual alkaline earth. U.S. Pat. No. 2,731,355 discloses a magnesium aluminum silicate and attributes the superior qualities thereof in part to the absence of calcia. Morrell, R. in Proceedings of the British Ceramic Society, No. 28, pages 53-71, June 1979, reported low theoretical densities of bodies prepared by adding 0.2 and 0.4 percent calcia to glass powder having the composition of cordierite. U.S. Pat. No. 4,367,292 discloses a process for the preparation of a powder which upon firing is converted to cordierite, and that the presence of impurities, particularly calcium oxide and alkalis, has an adverse effect upon the ability of the material to resist thermal shocks.
In the preparation of cordierite bodies from molten glass to produce glass-ceramic bodies, it is common to employ additives. For example, U.S. Pat. No. 3,450,546 discloses the preparation of transparent sintered glass-ceramic articles, having .alpha.-cordierite as the principal crystalline phase, produced by mixing particulate glasses consisting essentially of magnesia, alumina and silica. However, it is further disclosed that the base glass composition is compatible with small amounts of certain metal oxides which are beneficial as melting aids, and which improve the clarity of the product. It is taught that the total of these extraneous oxides should not exceed 10 percent by weight, the oxides being CaO, SrO, BaO, La.sub.2 O.sub.3, PbO, P.sub.2 O.sub.5, ZnO and B.sub.2 O.sub.3. Similarly, U.S. Pat. No. 3,480,452 teaches the preparation of a crystalline glass-ceramic material formulated from two glass frits, the first frit consisting substantially of silica, alumina and magnesia, and a second bonding frit consisting substantially of silica, alumina, magnesia and from 3.6-8.7 percent each of calcia and baria. U.S. Pat. No. 4,451,516 discloses a ceramic article comprising a plurality of ceramic parts bonded into a monolithic structure using glass-ceramics consisting mainly of magnesia, alumina, silica, 0.1-3.0 percent of BaO and 0.01-1.0 percent of ZrO.sub.2. It is further disclosed that the bonding glass may contain CaO, TiO.sub.2, Fe.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O, MoO.sub.3, B.sub.2 O.sub.3, CuO and the like in a total amount of not more than 10 percent.
It would be desirable to have a cordierite body having very low impurity levels and high flexural strength. The low levels of impurities would be desirable in that the cordierite body would have a low dielectric constant similar to that of undoped cordierite. The high strength would be desirable in that it would allow the use of thinner parts, would allow use in applications in which high resistance to stress is required, and would impart high thermal shock resistance to the cordierite body.